1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave resonator and a surface acoustic wave filter, and a method for manufacturing the surface acoustic wave device, and more specifically, the present invention relates to a method for manufacturing a surface acoustic wave device provided with high-power durable electrodes for high-frequency operation and high-power operation.
2. Description of the Related Art
In recent years, surface acoustic wave devices including a desired electrode pattern disposed on a piezoelectric substrate have been widely used for surface acoustic wave filters and surface acoustic wave resonators with the development of mobile phones and mobile communication systems. The electrodes of the surface acoustic wave devices are made of aluminum from the outset of development due to a low specific gravity and a low electrical resistance of aluminum. On the other hand, a surface acoustic wave device designed for high-frequency operation requires thinning electrode layers and miniaturizing an electrode pattern. The electrode layers on a piezoelectric substrate of a surface acoustic wave device are subjected to repeated stresses that depend on the frequency applied thereto during operation. Further, the stresses applied to the electrode layers constituting the surface acoustic wave device increase with an increase in operating power and frequency. These applied stresses cause migration of aluminum atoms of the thinned and miniaturized electrode layers of aluminum, resulting in the occurrence of defects in the aluminum electrode layers. There is a problem that the defects significantly impair the properties of the surface acoustic wave device.
Measures for enhancing the power durability and the reliability of the electrode layers by improving the orientation and controlling the crystallinity of the aluminum layers are known. These measures are significantly affected by the surface condition of the substrate used to construct the surface acoustic wave device. As a result, these measures have problems in which the aluminum layers have insufficient stability. Moreover, these measures also have problems in which the aluminum layers have insufficient properties suitable for further increased high-frequency operation and high-power operation.
Thus, a method for manufacturing a surface acoustic wave device that has electrodes made of single-crystal aluminum or an aluminum-based alloy disposed on a substrate made of lithium tantalate (LiTaO3) or lithium niobate (LiNbO3) is disclosed in Japanese Unexamined Patent Application No. 8-195635. This conventional method includes a process for removing impaired surface layers of the substrate prior to forming the electrodes made of the single-crystal aluminum or the aluminum-based alloy, the process including an ion implantation step, a chemical etching step, and an oxidation step, the steps being performed in that order.
However, surface acoustic wave devices disclosed in the known art described above have the following problems.
Japanese Unexamined Patent Application No. 8-195635 discloses that pretreatment for achieving a desired surface condition of the substrate before forming the electrode layers includes the steps of ion implantation, chemical etching, and oxidation. In the ion implantation step, the surface of the substrate is treated with an ionized inert gas or ionized nitrogen. This step requires a vacuum unit having desired functions. The substrate is subjected to the chemical etching, followed by the oxidation. This oxidation is performed with, for example, aqueous hydrogen peroxide, ozone, oxygen plasma, or oxygen radical. As a result, the oxidation step requires a unit having desired functions.
Consequently, many steps and a plurality of units for the treatment are required to achieve a desired surface acoustic wave device. Thus, the method described in Japanese Unexamined Patent Application No. 8-195635 has a problem in which manufacturing cost is high. The method also has another problem in which power durability is insufficient for further increased high-frequency operation and high-power operation.